US20110029087A1 - Intervertebral prostheses with compliant filler material for supporting adjacent vertebral bodies and method - Google Patents

Abstract

An intervertebral prosthesis and method includes an artificial interbody disc for engaging faces of adjacent natural vertebral bodies when a natural disc is replaced. Channels in the interbody disc can provide flexible characteristics to accommodate loads on the subject's spinal column. A bio-compatible filler material having a flexural stiffness of less than 12 GPa can encourage bone growth in the channels. The filler material may be inserted during an operation or pre-inserted in the disc channels during manufacturing of the disc.

Description

CROSS-REFERENCE TO RELATED APPLICATION
• This application is a continuation-in-part application from PCT/US06/47902 filed on Dec. 14, 2006 and also claims priority from U.S. Provisional Application Ser. No. 61/072,987 filed on Apr. 4, 2008.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• This application relates to a method and an interbody disc with compliant natural and/or artificial filler material for restoring spinal motion between vertebral bodies between which a natural spinal disc has been removed in whole or in part.
• 2. Description of Related Art
• In the field of spinal surgery, many treatment options exist to treat spinal pain, nerve impingement and spinal instability where a natural disc has failed in whole or in part. One such treatment is the removal of a damaged disc and its replacement with an intervertebral spacer which promotes fusion of bone between the separated vertebral bodies. This type of procedure when successfully completed, will result in a large bone mass between the vertebral bodies which will stabilize the column to a fixed position. SeeFIG. 1 where adjacentvertebral bodies7 and8 have been bridged with a solid mass of fused bone9. This procedure is hereinafter referred to as rigid fusion. Also, see U.S. Pat. No. 6,447,547 to Michelson which discloses a spinal disc spacer intended to be infused with a solid, relatively motionless mass likeFIG. 1. However, the success of a rigid fusion procedure appears to be one of many causes of adjacent segment disease. The lack of motion and the transfer of energy through the rigid fusion forces the adjacent intervertebral structures to adjust to the higher loads and motions or fail. Adjacent segment disease occurs as they fail.
• Ball and socket type disc arthroplasty devices have been tried for over 30 years. See U.S. Pat. Nos. 5,676,701 and 6,113,637. Their design rational is to allow motion in the hopes of reducing higher loads to adjacent structures. These have shown some success but also failures. A ball and socket type device requires no energy to rotate. Thus, the work absorbed by the device during rotation is zero. The rotation centers may be favorable at one specific instantaneous center of rotation present in a natural healthy disc, but is never correct nor favorable for all movements. This forces abnormal loads on adjacent structures. Materials needed for a stable ball and socket device are often very stiff or incompressible, thus any axial loads and especially shock loads through the device are almost completely transferred to the adjacent structures. A patient expecting a favorable outcome with a ball and socket lumbar disc arthroplasty device may find unfavorable results if repeated axial loads I shocks (along the spine axis) are a common occurrence.
• U.S. Pat. No. 4,309,777 to Patil, discloses an artificial disc with internal springs intended to flex. The device relies solely on the internal springs to provide the mechanical flexing motion. U.S. Pat. No. 5,320,644 to Baumgartner, discloses a different type of a mechanical flexing device. This device uses overlapping parallel slits forming leaf springs, which may contact in abrupt load paths, yielding impact stress. U.S. Pat. Nos. 6,296,664, 6,315,797 and 6,656,224 to Middleton, attempt to solve the disadvantage of abrupt load paths with a device containing a pattern of slits to allow for a more continuous load path. Middleton's device further includes a large internal cavity defined by the exterior wall. The internal cavity may be packed with bone to rigidly fuse adjacent vertebral bodies or capped with opposing plugs which limit the device's motion. Middleton's devices are intended to have a continuous load path with no abrupt load stops. These devices must be sufficiently stiff to support the anatomical average and extreme loads, thus too stiff to provide soft fusion as defined hereinafter.
• U.S. Pat. No. 6,736,850, to Davis, discloses a pseudoarthrosis device containing small (0.25 to 2 mm inner diameter), flexible, permeable material tubes as to allow fibrous ingrowth. This device is very soft and may collapse under normal loads and may likely not form bone within the small inner diameters.
• See published application nos. US20060217809A1; US20060200243A1; US20060200242A1; US20060200241A1; US20060200240A1; and US20060200239A1. It is the apparent attempt of the intervertebral prosthetic discs disclosed in these latter publications, to restore full intervertebral motion. However, these devices, as a result of their design, may be soft and very flexible resulting in artificial discs capable of absorbing little energy when subjected to shock loads. Computer simulations and mechanical validations of discs obviously patterned after some of these designs showed that it takes minimal loads (e.g., less than about 5 lbs for the cervical and less than 20 lbs for the lumbar) to compress the devices. While the weight required to be supported by an individual's spinal column will, to a great extent, depend on the individual's size, the weight to be supported in the cervical, thoracic and lumbar regions, will range from about 5 to 30 lbs, 30 to 60 lbs and 60 to 150 lbs or more in the cervical, thoracic and lumbar regions, respectively. Computer simulations also demonstrated that the use of a spiral slot or slit extending from the outer to the inner wall and encircling the disc two or more times as is illustrated in some of the publications is probably the reason for this lack of stiffness. A device which is too soft, will fully collapse when the patient is vertical, allowing for no additional movement to absorb impact energy. These types of soft spring devices, believed to have a stiffness of about 2.0 newtons(N)/mm, for use in the cervical region, and about 22.0N/mm in the lumbar region. Some of the patents/publications do show a vertical hole in the device, but apparently it came about for manufacturing purposes not for functionality. These patents do not describe or imply an intended fusion.
• Several of the above references disclose the use of mechanical springs or bellows as the means to separate adjacent vertebrae while providing movement therebetween during flexure and extension. Such spring arrangements, beside their other problems, such as fracture at attachment points to end plates, provide little shock and energy absorption capability because they either fully compress at normal loads, or fracture at high loads.
• There is a need for an intervertebral disc replacement or spacer for simulating the motion and energy shock absorption characteristics of a natural disc. To this end our novel intervertebral disc and method relies on a combination of mechanical flexure elements and bone and/or soft tissue infiltration within the disc to accommodate such motion and compliant filler materials such as a mixture of natural bone and/or artificial material for infiltration within the disc to accommodate such motion and energy absorption.
SUMMARY OF THE INVENTION
• Overview
• A desirable condition, which we term soft fusion, can be created between a patient's adjacent vertebral bodies in which the natural disc has failed in whole or in part by a) removing the failed disc or failed portion thereof; b) installing an artificial intervertebral disc between the two vertebral bodies; c) the disc providing one or more selected continuous or discontinuous channels of limited size for bone to form and fuse into one or more continuous or discontinuous struts between the vertebral bodies; d) the device being stiff enough to support the bodies in their natural spaced relationship while allowing limited motion and flexible enough to transfer sufficient energy to the bone struts to create one or more conditions of nonunion joints or pseudoarthrosis resulting in living nonrigid bone growth; and e) the disc being further arranged to limit its movement to an amount which is sustainable by the disc without resulting in fatigue failure during an anticipated lifetime.
• The cortical/cancellous bone of a vertebrae, particularly in the lumbar region, is very stiff. For example, avertebral body 30 mm in diameter with cortical bone around the outer 5 mm and cancellous bone (softer bone) on the inner area, which is 25 mm in height, will have an axial stiffness of approximately 235,000 N/mm or 235 KN/mm. The stiffness (axial) of a disc enabling soft fusion in accordance with the present invention should be between about 50 to 4000 N/mm, preferably within the range of about 200 to 1500 N/mm and most preferably between about 400-800N/mm. The size of the bone accommodating channel(s) should occupy about 10-35% (or less) and preferably about 12% to 25% of the total area of the disc facing the vertebral body to be supported.
• A condition of soft fusion is illustrated inFIG. 2 where acentralized bone strut6 of limited dimensions is allowed to form within an open core orbone channel17 of an interbody disc orspacer10 to be described in more detail in connection withFIGS. 3-10.
• Thebone strut6, extending between the vertebral bodies, has formed regions of pseudoarthosis or nonunion locations6a.The nonrigid bone struts along with the mechanical properties of the artificial disc accommodate additional energy absorption with increased movement per given load simulating, to a significant extent, the performance of a natural disc.
• Preferably in addition to the inclusion of an open continuous or discontinuous core(s)17 to accommodate the bone strut(s) the spacer will include generally horizontally oriented tissue accommodating channels (“tissue channels”)22,24 to promote vascularization and fibrous tissue ingrowth.FIG. 2aillustrates vascularization taking place within the tissue and bone channels. We refer to a disc which enables soft fusion as well as accommodates soft tissue infusion such as is demonstrated inFIG. 2 as a hybrid disc or device.
• The added advantage of tissue channels in conjunction with the bone strut forming channel(s) is that upon each loading and unloading cycle of the spine, nutrients and cellular waste will be pumped through tissue channels forming fibrous tissue within the tissue channels (vascularization). The nutrients and cellular waste are also pumped in and out to the bone strut(s). The disc may be “tuned” to match the deflection per load ratios to that of a natural healthy disc. The additional benefit of the soft tissue vascular areas (or bone void areas) is that soft tissue provides little initial resistance to compression but provides increasing resistance to an increasing compressive load. The natural disc is also softer at lower compressions than at higher compressions (axial or bending). A soft fusion device, infiltrated with adequate soft tissue in the tissue channels or voids will produce a device which is nearly as soft as the implanted device or natural disc, when subjected to light loads, and then become stiffer with increased compression or bending, just as a natural disc will. This unique ability of a soft fusion device with applied vascular cellular inputs promotes a device which will closely mimic a natural healthy disc embracing the ability for the soft tissue to heal due to vasculization or to fuse upon a lack of device motion due to non use or device collapse or flexural element(s) failure.
• A soft fusion device may take on many different forms and structures which will be as individualized as the anatomical location, desired outputs, and designer preferences but encompass the spirit of the invention. Obviously, the device must have a stiffness less than that of bone, but sufficient to maintain the supported vertebrae in a desired spaced relationship when the spine is subjected to light loads and flexible enough to transfer sufficient energy to bone strut(s) to create nonunion joints6awhen the spine is subjected to additional loading. Bone growth between the vertebral bodies outside of the selected bone accommodating channels is to be inhibited by limiting the available void volume, orientating the voids in a direction generally tangent to load paths, adding cellular inputs to specific void areas, filling the voids with a fluid or softer material and/or other means. The bone channels shown in this application are generally vertical and generally continuous. This is not a requirement for a soft fusion device. The device may have multiple channels in varying directions which do not need to be continuous. A discontinuous bone channel or an interrupted channel may extend ⅓ the total device height from one vertebral body and ⅓ the total distance from the opposing vertebral body and the device may be interrupted within the middle ⅓ of the device, for example. A channel extending at an angle from the endplate, at 60 degrees from vertical for example, may be useful to in allowing for more axial compression than a vertical channel. All these variations are allowable and in the spirit of a soft fusion device.
• It may be possible for an artificial spinal prosthesis or disc to accomplish the same degree of limited motion, load dampening, and energy absorption of a soft fusion device but without the living bone struts (and preferably soft tissues layers) created by soft fusion, it will not have the unique ability to adapt to the patient's loading conditions, repair itself when broken, and have the unique ability to fully support the vertebral column in the unlikely event that the underlying interbody disc fails.
• It is to be noted that the creation of a soft fusion state after the installation of a soft fusion hybrid device, in accordance with this invention, is dependent upon a patient's level of activity. For example, if a patient is sedimentary, i.e., moves very little, the bone formed with the channel(s) will become dense and rigid limiting the motion and energy absorption while protecting the spinal column stability. If the patient is more active, i.e., subjecting the struts and the device to additional loads, e.g., walking, lifting, etc., the bone core(s) will be less solid, i.e., fractured, not fully formed and/or infiltrated by soft tissue, allowing for more motion and energy absorption. This type of soft fusion/hybrid device will be able to change throughout the life of the patient, just as the body is able to remodel for given inputs. If the mechanical dampening and flexible members of a soft fusion device fatigue, crack and fail, the device will slightly collapse. The collapse will limit the motion and eliminate the dampening action of the device thus transferring the energy to the supporting bone strut(s), promoting additional bone fusion and support.
• Mathematical Rational
• The theory behind soft fusion may be best understood by analyzing only the differences between a soft and rigid fusion rather than attempting to analyze actually true loads, deflections, and energy absorption capabilities. This is done by starting with basic equations.
• Axial Deflection (δ) in the cephalic/caudal direction is equal to,
• $\begin{array}{cc}\delta =\frac{\mathrm{PL}}{\mathrm{AE}},& \left(\mathrm{Eqn}.1\right)\end{array}$
• where P is the applied force, L is the length of the strut (disc height), A is the cross-sectional area and E is the modulus of elasticity.
• Bending curvature
• $\left(\frac{1}{\rho }\right)$
• either in flexion/extension or lateral bending is equal to:
• $\begin{array}{cc}\frac{1}{\rho }=\frac{M}{\mathrm{EI}}& \left(\mathrm{Eqn}.2\right)\end{array}$
• where M is the applied bending moment and I is the moment of inertia.
• Soft fusion works by displacing under applied forces more than possible with a ridged fusion. Strain energy (U) is defined as the energy uptake or energy absorbed by the deformation of the material by the applied load or:
• 
  U=∫₀^x1P*dx  (Eqn. 3)
• where P in an applied force and the integral of x from 0 to x1 is the deformation. Deformation noted inequations 1 and 2 may be inserted intoequation 3 to determine the actually strain energy.
• Many assumptions must be made to analyze the forces and deflections through the vertebral column and associated structures in order to accurately determine strain energy or energy absorption. However, the validity of soft fusion may be proven by simply comparing the variables unique between soft and rigid fusions. For the abovementioned device these are 1) the cross-sectional area of the bone strut verse the cross-sectional area of a rigid fusion 2) the presence or absence of the device in conjunction with the bone strut and for sake of comparison to arthroplasty ball and socket devices, 3) the modulus of elasticity.
• To first look at the axial energy absorbed with the first set of variables we only need to define the cross-sectional area of a soft fusion as approximately 0.785 cm̂2 and the cross-sectional area of a rigid fusion as 15.4 cm̂2. These are typical cross-sectional areas seen within the lumbar region. By then setting the strain energy of a soft fusion to U_Sand that of a rigid fusion to U_Rthe relation between the two becomes:
• $\begin{array}{cc}\frac{U_s}{U_r}=\frac{{\int }_0^{\delta n}P*\delta r}{{\int }_0^{\delta r1}P*d\delta r}& \left(\mathrm{Eqn}.4\right)\end{array}$
• With equal assumptions to both soft and rigid fusions and with all variables except the cross-sectional areas equal, equation 4 becomes:
• $\begin{array}{cc}\frac{U_s}{U_r}=\frac{\frac{1}{\mathrm{As}}}{\frac{1}{\mathrm{Ar}}}=\frac{\mathrm{Ar}}{\mathrm{As}}=\frac{15.4{\mathrm{cm}}^2}{\mathrm{.785}{\mathrm{cm}}^2}-19.6& \left(\mathrm{Eqn}.5\right)\end{array}$
• In other words, a fully formed soft fusion bone channel will absorb 19.6 times more axial energy than a rigid fusion based solely on the area of available bone. The soft fusion device will reduce this number to some degree, depending on the stiffness of the actual device. Such constricted bone growth should not fully form in active patients or become fractured with high patient generated forces. When this occurs the presence of nonunions and fibrous tissue within the defined strut location(s) will only aid the soft fusion energy absorption capabilities by softening the hybrid bone, tissue, and implanted device creating a condition of a controlled pseudoarthrosis.
• By neglecting the minimal effects of the Soft Fusion device and only comparing the bone strut to a cobalt chromium ball and socket device we see that the strain energy relationship in axial compression is approximately equal to:
• $\begin{array}{cc}\begin{array}{c}\frac{U_{\mathrm{SoftFusion}}}{U_{\mathrm{CoCr}}}=\frac{\frac{1}{\mathrm{As}*E_{\mathrm{bone}}}}{\frac{1}{A_{\mathrm{disc}}*E_{\mathrm{COCR}}}}\\ =\frac{A_{\mathrm{disc}}*E_{\mathrm{COCR}}}{\mathrm{As}*E_{\mathrm{bone}}}\\ =\frac{15.4{\mathrm{cm}}^2*220\mathrm{GPa}}{\mathrm{.785}{\mathrm{cm}}^2*17\mathrm{Gpa}}\\ =253\end{array}& \left(\mathrm{Eqn}.6\right)\end{array}$
• As seen inequation 6, a cobalt chromium articulating device is extremely poor at absorbing axial impacts.
• Similar bending calculations are currently omitted because of their redundancies to this application but would show similar results.
• Suitable Intervertebral Disc Structure for Enabling Soft Fusion
• A preferred intervertebral motion restoring disc for supporting adjacent vertebral bodies in their natural spaced relationship after a natural disc has been partially or wholly removed in accordance with the present invention has upper and lower surfaces for engaging the faces of the vertebral bodies to be supported and a support structure between the surfaces having a stiffness within the range previously discussed. The disc defines one or more generally vertically oriented continuous or discontinuous bone growth channels of limited cross-sectional area enabling bone struts to form therein extending at least partially and preferably completely between the bodies. The disc (with its stiffness characteristics) and the resulting bone strut or struts are arranged so that predetermined axial and/or bending loads thereon, e.g., normal loads, loads associated with standing or walking, will not fully compress the disc allowing a narrowing of the distance between the supported bodies during normal motion and create one or more pseudoarthrosis or fibrous nonunion locations along the length of the strut(s) to provide soft fusion thereby limiting a complete rigid strut formation. The disc further fully compresses at predetermined excessive forces in order to protect the flexural members of the device from overloading and failure. The unique combination of one or more pseudoarthrosis bone struts and the mechanical disc supporting structure results in the condition of soft fusion as previously discussed. Such controlled and limited fusion, i.e., soft fusion, provides limited motion, both translational and rotational and energy and shock absorption characteristics surpassing that of a rigid fusion while preserving vertical column stability.
• First, vertebral column stability is particularly important in that it prevents disc induced or allowed kyphosis and scoliotic curvatures as seen with ball and socket type devices. Some prior art articulating devices will often settle into a fully rotated position when the soft tissue is unable to stabilize the spinal column. A soft fusion disc provides a force towards the central position assisting to stabilize the spinal column. Second, disc stability is important in that the continuous or discontinuous bone channels will likely form some degree of bone with soft tissue infiltration. This will greatly aid in preventing device expulsion, a failure mode seem with other non-fusion devices.
• One such intervertebral disc acceptable for providing soft fusion and particularly designed for anterior insertion in the lumbar/thoracic region includes a pair of end plates (or layers) with each end plate having an outer intervertebral engaging surface for buttressing against a respective vertebral body and an inner surface. A plurality of interleaved first and second axial dampening plates (or layers) are sandwiched and secured between the inner surfaces of the end plates.
• Each of the individual dampening plates define a peripheral outer wall and an inner generally cylindrical open bone accommodating core aligned along a longitudinal axis which will be generally aligned with the patient's spinal column when installed. Every other pair of axial dampening plates may be bonded, e.g., welded, together adjacent the inner core (or machined) leaving a generally planar space therebetween extending outwardly from the bonded area beyond the outer walls. The remaining pairs of axial dampening plates may be bonded, e.g., by welding, together along their peripheral walls (or machined) leaving a generally planar space therebetween extending from the bonded area to the open core. This arrangement provides alternating spaces extending from the core outwardly and from the peripheral walls inwardly which allows the end plates and the vertebral bodies to which they are secured to have limited translational motion parallel to the longitudinal or spinal axis and limited pivotal motion about the axis while dampening both motions. The channels formed between the plates and particularly the channels extending inwardly from the peripheral wall will accommodate tissue infusion and function as tissue channels.
• Preferably the dampening plates are provided with one or more flexion slots between the outer peripheral walls and the inner cores to provide increased flexing action. The periphery of plates preferably follow the contour of the disc which they are to replace, e.g., an outer, generally convex, peripheral wall merging with a generally concave inner wall. As an option, a rotational dampening subassembly, to provide limited rotational motion between the end plates, can be inserted into the sandwiched axial dampening plates. Such assembly comprises an inner generally circular planar torsional dampening spring member with a helical slot, mounted between upper and lower torsional plates so that one of the torsional plates can rotate through a limited angle relative to the other. Alternatively, the spacer may be formed with about a 1½ turn or helical slot extending from the exterior wall to the central core(s) eliminating the interleaved plate construction as will become apparent in reference to the appended drawings.
• The plates may be made of a suitable biocompatible material such as a titanium, cobalt or stainless steel alloy and or super elastic metals, e.g., nitinol, which in the sandwiched assembly, has sufficient strength and flexibility (stiffness) to withstand the anticipated stresses while providing the desired motion requirements to allow nonrigid bone struts to form within the open core.
• In one method of construction the assembly is built plate by plate (or layer by layer) with the individual plates joined by diffusion, laser or electron beam welding or perhaps with a mechanical interference fit only.
• The assembly may be constructed in various configurations adapted to site specific in vivo locations such as anterior, anterior lateral, lateral, lateral posterior or posterior spinal interbodies, interspinous dampening spacers, interconnecting pedicle screw dampening members or other posterior element stabilization devices.
• An intervertebral disc particularly designed for the cervical region of the spine is formed with upper and lower surfaces for engaging the respective vertebrae faces to be supported and a generally elliptical partially obstructed open core for accommodating the formation of one or more bone struts. The spacer includes generally planar semicircular soft tissue integration channels extending inwardly from a peripheral wall to a location short of the open core. The tissue channels are interleaved with planar channels extending outwardly from the core to a location short of the peripheral wall.
• While providing various examples of intervertebral prosthetic discs and a method for accommodating the creation of soft fusion within the discs advances the state of this art, we now propose improvements to provide a superior prosthesis and method by filling the channels defined in the intervertebral prosthetic discs with a material which is less stiff than typical cortical bone including some cancellous bone used in the prior art devices. For example, a filler material having a flexural stiffness less than 10-12 GPa will improve the load compliance and flexibility of the intervertebral prosthesis.
• Where a cortical/cancellous bone blend is to be used as the filler material, cancellous bone, which has a GPa of the order of 4 GPa, should comprise at least the predominate, if not, the sole constituent of the blend. This flexural stiffness is reported to be the average for cancellous bone. The use of such softer filler materials in the discs will allow for a more compliant and energy absorbing device even in the absence of a nonunion joint or pseudoarthosis. A softer filler material will in effect alleviate the need for a nonunion or pseudoarthosis by the formation of a more compliant yet stable fusion.
• An interbody disc, in accordance with the present invention, has (a) upper and lower surfaces for engaging the faces of the adjacent vertebral bodies between which a failed natural disc has been partially or wholly removed, (b) an exterior wall and one or more generally vertically oriented continuous or discontinuous channels (c) a sufficient stiffness to support the separated vertebrae in substantially their naturally spaced relationship while allowing limited motion and flexibility when subjected to a predetermined load to alter the distance between the vertebrae and thereby transfer load and energy to the any material filling the channels or voids and (d) a bio-compatible filler material disposed within the channels, the filler material being compliant and softer than cortical bone, e.g., having a flexural stiffness of less than about 10-12 GPa. The filler material combined with the device characteristics—will dampen the loads and energy transfer prior to the device contacting on the internal stops which will then in turn prevent fatigue failure.
• Where human bone is selected as the filler material, cancellous bone is the first choice. As a second choice cancellous and cortical bone can be blended with cancellous bone being the predominate portion of the blend such as a ratio of cancellous to cortical bone within the range of about 80% to 20% and preferably about 60%+ to 40%.
• Other naturally harvested materials (either from the patient, a donor or an animal) suitable for use in the blend can include any substances softer than bone, such as portions of the removed disc. Morselized bone or bone weakened by gamma sterilization is more compliant then cortical bone and may also be useful as a filler material.
• Bone graft substitutes, such as demineralized bone matrix (DBM), calcium sulfate dehydrate (CSD) ceramic-based bone graft extenders, are believed to be satisfactory filler materials. These will have a very low flexural modulus to allow device bending characteristics but will resist compressive forces when contained in a generally vertical channel. Recombinant Human Bone Morphogenetic Protein (rhBMP-2) liquid, Epidermal Growth Factor (EGF) liquid, Platelet Derived Growth Factor (PDGF), Fibroblast Growth Factors (FGFs), Parathyroid Hormone Related Peptide (PTHrp), Insulin-like Growth Factors (IGFs), and Transforming Growth Factor-Betas (i.e., TGF-B1), may also accompany suitable filler materials in order to induce a specific biological response such as bone or soft tissue activity. Another filler material candidate is polyetheretherketone (PEEK) with or without porosity. This synthetic material has mechanical properties very similar to those in cortical bone without porosity and very similar to cancellous bone when used with porosity. In addition, it is highly controllable and stable allowing the disc to be preassembled with the filler material at a factory site.
• The method of the present invention entails a) providing a prosthetic disc as discussed above, b) filling the channels with the appropriate filler material, either at the surgical site or at a manufacturing site, c) removing the damaged or failed disc in whole or in part, and d) inserting the filled disc between the separated vertebral bodies.
• The structure and function of an intervertebral disc for creating soft fusion and method for accomplishing this desired condition are explained in more detail in the accompanying description of the preferred embodiments taken in conjunction with the appended drawings wherein like components (or locations) are given the same reference numerals.
BRIEF DESCRIPTION OF THE DRAWINGS
• The objects and features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages, may best be understood by reference to the following description, taken in connection with the accompanying drawings.
• FIG. 1 is a side elevational view of adjacent vertebral bodies separated by a conventional rigid fused bone mass;
• FIG. 2 is a perspective view, partially in cross-section, of adjacent vertebral bodies separated by an interbody disc and a central bone strut containing fibrous nonunion locations forming a soft fusion disc;
• FIG. 2ais an enlarged partial view of the disc ofFIG. 2 showing vascularization taking place within the tissue/bone channels;
• FIG. 3 is a perspective view of an anterior interbody disc in accordance with the present invention;
• FIGS. 4 and 5 are plan and front views, respectively, of one of the outer or first dampening plates of the disc ofFIG. 3;
• FIGS. 6 and 7 are plan and front views, respectively, of one of the inner or second dampening plates of the disc;
• FIG. 8 is a front elevational view of the assembled plates ofFIGS. 4-7;
• FIGS. 9 and 10 are top plan and front views, respectively, of the disc;
• FIG. 11 is a cross-sectional view of the disc ofFIG. 1 taken along lines11-11;
• FIG. 12 is a front elevational view of the disc showing articulated/pivotal motion between the end plates about the longitudinal axis;
• FIGS. 13 and 14 are top plan views, respectively, of the upper and lower torsional plates of a torsional dampening subassembly;
• FIG. 15 is a plan view of an inner torsional dampening spring member of the subassembly;
• FIG. 16 is a front view of the assembled torsional dampening subassembly;
• FIG. 17 is a cross-sectional view of the device ofFIG. 3 incorporating the rotational dampening subassembly, taken along lines11-11 ofFIG. 3;
• FIG. 18 is a cross-sectional view of the device ofFIG. 3 incorporating the rotational dampening subassembly taken along lines18-18;
• FIG. 19 is the same cross-sectional view asFIG. 17 showing articulation of the device;
• FIG. 20 is a perspective view, partially broken away, of a modified disc similar to the disc ofFIG. 3 showing the migration of bone within the central core and soft tissue within the soft tissue channels or voids;
• FIG. 21 is a cross-sectional view showing the disc in a vertically compressed mode;
• FIG. 22 is a graph showing a typical moment versus rotation plot of a natural disc versus that of a computer model of the artificial disc ofFIG. 20 when bone and tissue have penetrated the voids as illustrated inFIG. 20;
• FIGS. 23 and 24 are perspective and side elevational views, respectively, of an alternative embodiment of a disc primarily designed for the cervical region;
• FIGS. 25 and 26 are cross-sectional views taken along lines25-25 and26-26, respectively inFIG. 23;
• FIGS. 27-29 are top plan, side elevational and cross-sectional views of other interbody discs for providing increased rotational mobility;
• FIG. 29ais a cross-sectional view of the disc ofFIGS. 27 and 28 as modified to eliminate the threaded connection and provide a gap between the exterior walls of the upper and lower sections;
• FIGS. 30 and 31 are side elevational and cross-sectional views of another interbody disc;
• FIGS. 32-34 are top plan, side elevational and cross-sectional views of another disc embodiment;
• FIGS. 35-37 are top plan, side elevational and cross-sectional views of another disc embodiment;
• FIGS. 38-42 are top plan, side elevational, bottom, end and cross-sectional views, respectively, of a base component of an alternative two-piece disc;
• FIGS. 43-47 are top plan, side elevational, bottom, end and cross-sectional views of an upper component of the two piece disc;
• FIGS. 48-51 are top plan, side elevational, bottom and cross-sectional views of the assembled two piece disc;
• FIGS. 52-56 are top plan, side elevational, bottom end and cross-sectional views of an upper component of an alternative two-piece disc;
• FIGS. 57-61 are bottom plan, side elevational, top, end, and cross-sectional views of a base or bottom component of the two-piece disc;
• FIGS. 62-64 are side elevational, end and cross-sectional views of the assembled two piece disc;
• FIGS. 65-68 are top plan, side elevational, end and cross-sectional views of another embodiment of an anterior disc;
• FIGS. 69-72 are top plan, side elevational, end and cross-sectional views of a posterior disc in accordance with the invention;
• FIG. 73 is a perspective view of two of the discs ofFIGS. 69-72 placed on the exposed face of a lower vertebral body;
• FIGS. 74-78 are top plan, side elevational, front end, rear end and cross-sectional views of an alternative posterior disc;
• FIGS. 79 and 80 are a perspective view and a cross-sectional view respectively, taken along the length of the disc, insitu between vertebral bodies, with bone growth shown;
• FIG. 81 is a cross-sectional view, insitu between vertebral bodies of a sheep with bone growth shown;
• FIG. 21A is a cross-sectional view showing the disc ofFIG. 21 with filler material;
• FIG. 64A is a cross-sectional view showing the disc ofFIG. 64 with filler material; and
• FIG. 73A is a perspective view of the discs ofFIGS. 69-72 with filler material.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• Reference will now be made in detail to the preferred embodiments of the invention which set forth the best modes contemplated to carry out the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention.
• Referring now toFIG. 3 anupper end plate11, providing an outer orcephalad surface11afor buttressing against an upper vertebral body, is joined to alower end plate12, providing an outer or caudal surface12a(not shown inFIG. 3). A group of interlocking interior plates or layers, collectively referred to as13, are disposed between and joined to theinner surfaces11band12bto form an anterior interbody device or artificial disc. SeeFIG. 11. Theouter surfaces11aand12aof the end plates may be provided with an array of mechanical locking features such as thekeels14 or alternative geometric features and fixation rings15aand15b.The fixation rings may be constructed of an osteointegrative porous material which abut the edge of a hollow core orbone channel17. As discussed previously, thecore17 accommodates bone growth to form a continuous or discontinuous strut (with nonunion locations) adjoining the separated vertebral bodies.
• The fixation rings are stepped to provide additional purchase against the vertebral end plates and to fill the convex surface of the adjacent vertebral end plate. The core may be packed with bone to accelerate the formation of the strut or other material. Thecore17 and the interior plates layers14 may be, but preferably are not, shielded from surrounding tissues to prevent tissue integration or device particulate wear explosion. Dacron or polytetrafluroethylene are preferred material to provide device shielding if desired.
• FIGS. 4 and 5 illustrate a first axial dampeningplate18 which forms one of the interior interleaved plates of the assembly orgroup13. Theplate18 includes a centralcylindrical opening18a,a generally convex peripheralfront wall18b,merging with a generallyconcave back wall18c,a protrudinglip18d,extending along the periphery, and optional front andback flexion slots18fand18g.The opening18ais framed by acylindrical wall18hwhich is bonded to an inner shoulder of the second plate as will be explained.
• FIGS. 6 and 7 illustrate a second axial dampeningplate20. The second plates are interleaved with the first plates and disposed between and joined to the inner surfaces of theend plates11 and12 to form the motion restoring intervertebral device of the invention. Thesecond plate20 includes acentral opening20a,an upwardly protrudinginner shoulder20bsurrounding the opening, aperipheral wall20cin the form of front andback walls20dand20e,respectively. The second plates includeflexion slots20fand20gwhich align withslots18fand18gin the assembled device.
• FIG. 8 illustrates the first and second plates in an assembled condition with the first plate'sinner surface18hof the opening18abeing bonded to anouter surface20hof the second plate's protrudinglip20b.
• FIGS. 9 and 10 represent a top plan view and a front side view, respectively, of the assembled intervertebral device. It is to be noted that the second plates, when provided with anouter ring15a,can be used as the end plates, as is shown in the cross sectional view ofFIG. 10. The horizontal dashed lines represent the bond or weld between the peripheral walls of the first and second plates although the welds would generally not be visible in the finished device. X-X represents the longitudinal axis of the device. The term axial loads, as used herein, refers to loads or forces directed along the X-X axis. The alignedopenings20aof the second plates represent theopen core17.
• Referring now toFIG. 11, a cross sectional view taken along line11-11 ofFIG. 1, theperipheral lip18dof the first plates are bonded to a peripheral annular surface20ion one side of the second plates leaving a generallyplanar space22 extending inwardly from the bond or weld between18dand20ito theopen core17 as is shown inFIG. 9. Theannular surface18h,surrounding the opening18aof the first plate, is bonded to theouter surface20hof the next second plate to provide a generallyplanar space24 extending outwardly from the bond (i.e.,surface20h) to the outside of the device as illustrated. This pattern is repeated with the plates being assembled one plate at a time until all plates are stacked and welded. If the device is diffusion welded, all plates may be stacked and welded at one time. Thespaces22 and24 are preferably left open as shown and serve to allow the infusion of soft tissue as previously discussed.
• The intervertebral disc, with its interleaved plates, has motion yet sufficient stiffness or strength to support the vertebral bodies (7,8) in their natural spaced relationship while allowing limited motion and dampening the load applied to the bodies. When the separated vertebrae are subjected to normal loads, such as would be experienced by a person standing or walking, the plate will not be fully compressed allowing a narrowing of the distance between the vertebral bodies causing the bone strut formed in the core17 to fracture or form fibrous nonunion joints at one or more locations along its length. Greater loads, such as jogging or lifting heavy objects, will further aid this process of promoting nonunions.
• The fatigue life of the device is preserved by theinternal spaces22 and24 as shown inFIG. 11 which spaces are preferably left open to accommodate soft tissue ingrowth therein forming a hybrid device. The individual plates or layers may flex, bend (and/or rotate with an optional torsional dampening subassembly to be described) as designed until they deflect to a point collapsing the internal spaces or preferably compressing tissue infused therein. Once the internal spaces are collapsed with or without tissue therein, the individual plate's movement will be stopped by an adjacent plate. All plates or layers are designed so that movement within these internal spaces will not fatigue the material, thus preserving the fatigue life of the device.
• FIG. 12 is a front view illustrating an assembled intervertebral device under going articulation.
• FIGS. 13 and 15 are top plan views of theouter plates26 and28 which together with a spring30 (FIG. 14) form a torsional dampening subassembly shown inFIG. 16. Both plates follow the outside contour of the first and second plates and end plates. Theupper plate26 defines acentral opening26a(aligned along the longitudinal axis), a downwardly extending annular undercut26b(shown in dashed lines), and an upwardly extendingperipheral lip26c(like theperipheral lip18d). Thelower plate28 defines acentral opening28asurrounded by asurface28b.Atorsional spring member30 includesperipheral area30a,acentral opening30b,surrounded by an annular undercut30c(shown in dashed lines) inFIG. 13 and with an undercutedge30d,and aspiral slot30ewhich allows limited rotation between the dependingshoulder30cand theperipheral area30a.
• FIG. 16 shows the torsional dampening subassembly in its assembled form whileFIG. 17 illustrates a front cross-sectional view of the device ofFIG. 1 including the addition of the torsional dampening subassembly. Thelip26cof the upper plate is bonded to the second plate's peripheral lower surface20i.Theedge26dof the undercut26bis bonded to theperipheral area30aof thespring member30 with thesurface30dof thespring member30 being bonded to thesurface28bof thelower plate28. The lowerperipheral surface28cof thelower plate28 is bonded to the peripheral lip of the first plate as shown.
• FIG. 18 is a side cross-sectional view of the device ofFIG. 17 showing theflexion slots18f,18g,and20f,20g.
• FIG. 19 is a side cross-sectional view ofFIG. 17 illustrating articulated/pivoted motion between theend plates11 and12.
• The overall height h (FIG. 10) of the intervertebral motion restoring device will depend upon its selected location and the patient's anatomy. As an example, h should be within the range of about 0.19 to 0.315 inches and 0.315 to 0.8 inches for use in the cervical or thoracic and lumbar regions, respectively.
• As a further example for a height h of 0.565″ thespaces22 and24 (FIG. 5) are preferably 0.015 and 0.012 inches, respectively. Such a device may have a width w and a length t (FIGS. 4 and 5) of about 1.0 and 1.4 inches, respectively.
• A slightly modified disc is shown inFIGS. 20 and 21 in which corresponding components are identified with the same numerals.
• The difference between the intervertebral disc ofFIGS. 20-21 and the disc described inFIGS. 3-12 (except for the number ofintermediate plates18,20) is the addition of the extensions18iof central portions ofplates12 and18 which, when the spacer is in the unstressed condition,form gaps17bin the central section. These extensions serve as a stop means to limit the compression and axial articulation of the support structure without completely closing thechannels22 and24, preserving the fatigue life of the disc.
• As an example, for asupport structure13′ having a height of about 5 mm, thegaps17′bbetween the extensions18iand the adjacent plates may be about 0.015″ while thechannels22 and24 may have a height of about 0.020″. This difference in the dimensions of the gaps versus the height of the channels allows the spacer to be completely compressed (i.e., along the longitudinal axis) without completely closing thechannels22 and24 by providing stop means, i.e., contact locations along thecentral section17a,to accommodate abrupt loads and to alleviate fatigue failure which may otherwise occur as a result of repetitive loads. This also prevents complete soft tissue compression within thevoids22 and24 and allows for additional disc bending when fully compressed.
• FIG. 20 illustrates the infiltration ofsoft tissue19 within the channels between the plates and some migration of abone strut6 within thecore17 forming nonunions at locations6a.The soft tissue infiltration in the large areas within the channels results in a nonlinear increase in stiffness of the spacer as the load is increased thereby simulating the response of a natural disc. The design ofFIGS. 20 and 21, as well as the designs shown in subsequent figures have the ability to openly integrate with varying combinations and densities of bone and soft tissue, thus producing a hybrid device made of both inorganic (metal or polymer) and organic (cellular tissue and/or bone) materials. The advent of continuous or discontinuous bone struts through the device as discussed previously will yield a device which is stable, yet more flexible, than a device relying on rigid fusion, thus providing the capability of energy absorption. It is to be noted, however, that bone struts are not required to produce a positive result. Soft tissue, will in time, infiltrate the voids producing a device which will more closely mimic a natural disc. This will be explained in more detail in conjunction withFIG. 22.
• FIG. 21 illustrates the disc completely compressed by a vertically oriented or axial load with thegaps17′bclosed. The spacer in such a collapsed mode will still accommodate a lateral bending action, i.e., about the longitudinal axis.
• FIG. 22 is a graph showing lumbar-disc moment in Newton meters/degree verse rotation plot in degrees (around a horizontal axis) of a typical natural disc (curve34), hysteresis ignored. This response of a natural disc is illustrated by Dr. Spenciner D. et al., in The multidirectional bending properties of the human lumbar intervertebral disc. Spine J. 2006 May-June 6(3):348-57. The slope of the curve at any point represents the disc flexibility.Curves38 and36 represent the theoretical response of a computer model of the intervertebral spacer ofFIGS. 20 and 21. It is to be noted that the response of an actual mechanical disc made in accordance with this invention may vary from that shown by the curves. For small displacements and loads, soft tissue integration withinchannels22,24 will provide little resistance and thus the device will have a greater flexibility as shown bycurve38. As the motion and loads increase, the soft tissue will become increasingly compressed and strained. Due to soft tissues' nonlinear mechanical properties, the soft tissues will provide an increased degree of resistance with each increase in motion as is illustrated bycurve36. The actual hybrid Soft Fusion device flexibility curve will include a curve similar tocurve38 and transition to a curve similar tocurve36.
• The area of thebone accommodating core17 or cores should not exceed about 35% and preferably less than about 25%, (e.g., about 10-20%) of the total area of the disc facing the separated vertebral bodies, i.e., in a horizontal plane. The size of the disc and bone strut opening(s) therein will depend upon the size of the vertebral bodies to be supported. As an example, the total area of the openings should have a diameter, if circular, or equivalent dimensions if non-circular, within the ranges of 0.1 to 0.6, 0.1 to 0.7, and 0.2 to 0.7 inches in diameter for the cervical, thorax, and lumbar regions, respectively.
• An alternative embodiment of an intervertebral or hybrid disc designed primarily for the cervical region, is illustrated inFIGS. 23-26 wherein thedisc40 is formed with upper andlower surfaces40aand40b,respectively and a central elongated generally ellipticalopen core40c,partially obstructed by a bone integration diversion plate orbeam40hto be described.Keels40dextend outwardly from the upper and lower surfaces to aid in securing the spacer between the supported vertebral bodies. The spacer includes generally planar semicircular softtissue integration channels40eextending inwardly from the exterior orperipheral wall40fto a location short of theopen core40c.Generally semicircular tissue integration channels40gare interleaved with thechannels40eand extend outwardly from the core40cto a location short of the exterior wall. The centrally locatedbone diversion beam40hextends laterally across the core40cbelow theupper surface40aas shown more particularly inFIGS. 9 and 10.
• The beam is held in place by downwardly extending legs40iwhich are formed with or otherwise secured to the lower peripheral wall at40j(FIG. 23). The ends of thebeam40hare arranged to abut the opposed ends40kof the top of the spacer at the ends of the open core to limit the compression (and vertical articulation) of the spacer when subjected to excessive loads. It is to be noted that the number and configuration of the tissue integration channels may vary.
• Thebone diversion bar40hcreates channels401 (FIG. 25) which promote relatively narrow bone growth alonglines40mto result in soft fusion. The cross-sectional area at thechannel401 is preferably within the range previously discussed.
• Another alternative hybrid intervertebral disc42144 is illustrated inFIGS. 27-29 which includes an upper andlower section42 and44, respectively. Theupper section42 includes atop surface42a,an exteriorperipheral surface42b,aninner surface42csurrounding an opencylindrical core42d.A ring-shapedinner cavity42e,open at the lower end and forming an arch42fat the upper end is formed in the upper section. The upper section is also formed with a helical ¾-1½ turn slot (or channel)42gextending from the inner to the outer surfaces and through thecavity42eas shown inFIG. 29. The slot is formed with stress relievingend openings42h.The spiral slot42gaccommodates limited rotation about a vertical axis (e.g., about 3 degrees) and compression. The voids between the post and the cavity as well as the spiral slots accommodate the infiltration of soft tissue. Thehollow core42dwill accommodate the infusion of bone and or soft tissue growth.
• The inner surface42i(FIG. 29) facing thecavity42eis threaded at42kfor receiving thelower section44. Thelower section44 is formed with an upwardly extending annular or donut-shaped post44aextending into thecavity42e.The lower section includes male threads44boffset from an inner wall44cthereof which threads cooperate with thethreads42kto join the lower section to the bottom of the upper section as shown. An outwardly extendingflange44dabuts an annular shoulder42lto allow a surgeon to preset the compression of the spacer via thethreads42k/44bas will be apparent to those skilled in the art. The bottom surfaces44dand42nare arranged to engage the face of the lower vertebral body.
• The abutting surfaces44dand42lwill only transmit axial compressive and bending loads. This connection will only allow distractional, rotational and translational loads to be carried by the inner spring (formed by the inner cylindrical section42j), softening the device in those motions. Excessive translations will contact surfaces44cand42iand then load the outer spring (formed by the outer cylindrical section42m). The structure forming the inner and—outer springs is discussed in conjunction withFIG. 30.
• The upper end44eof the post44ais arranged to abut the top42fof the cavity to limit the compression and vertical articulation of the device.
• FIG. 29aillustrates a slight variation of the disc ofFIGS. 27-29. In this variation the threadedconnection42k/44bhas been replaced with a weld at44′fand the addition of asmall gap42′m,e.g., 0.010″ to 0.040″ between abuttingsurfaces42′land44′d.As a result the inner spring, formed by thespiral slot42′gin theinner wall42′n,takes substantially all of the compressive load until thegap42′mis closed. Then the outer spring, formed by the slot in theouter wall42′o,assists with resisting the forces. Thisgap42′malso serves another purpose. The outer spring accommodates only compressive loads (including bending), but not extraction or rotation about the longitudinal axis X-X. This arrangement softens the spacer for both loading conditions. The outer spring will also not absorb any translation until thegap42′mis closed. This will allow motion more closely simulating that of a natural disc.
• Another embodiment of a hybrid intervertebral disc46148 is illustrated in the side elevational and cross-sectional views ofFIGS. 30 and 31 with the understanding that the top plan view of the disc would be similar to that shown inFIG. 27. This spacer is formed with an upper section in the form of acylindrical hub46a,having aninner surface46bsurrounding an open central core46cand anouter surface46d.The hub extends upwardly from a flat bottom46eto an outwardlyflanged head portion46fto arim46g.Thelower section48 is in the form of an annular post48ahaving exterior andinterior surfaces48band48c,respectively, with the inner surface stair-stepped inwardly to formshelves48dand48ewith theshelf48eabutting the bottom46eof the upper section. The convex, i.e., semicircular,upper end48fof the post is arranged to abut theinner surface46hof theflanged head46fto stop the articulation of the hub when the device is subjected to excessive loads, while allowing limited rotation. The hub is formed with a ¾-1½ turn channel or slot46i.The top andbottom surfaces46kand48gare arranged to engage the faces of the respective vertebral bodies to be supported.
• The voids formed by the spinal slot and thespace49 between the outer and inner surfaces of the hub and the annular post, respectively, provide soft tissue ingrowth locations. The open core will allow bone and/or soft tissue ingrowth.
• FIGS. 32-34 illustrate an additional embodiment of the present invention in the form of an innercylindrical member50 having anopen core50aadapted for bone and/or soft tissue ingrowth and a centrally located 1-1½ turn helical slot50badapted for soft tissue ingrowth. An annularouter member52 includes abottom portion52awith itsinner surface52bsecured, e.g., by welding, to the outer surface50cof the inner member. The bottom50dof the inner member forms anannular shelf50ewhich sits under the bottom of theouter member portion52aas shown. Thetop portion52cof the outer member is secured at itsinner surface52dby welding, for example, to the outer surface along the top portion of the inner member. The top andbottom portions52aand52care formed with concave mirror image surfaces52eand52fbetween which an articulation stopping ring54 (circular in cross section), is positioned. The upper and lower surfaces of the50/52 disc serve to engage the faces of the supported vertebral bodies.
• The ring is preferably free floating within the space created by thesurfaces52eand52dand smaller in diameter than the distance between such surfaces to allow the inner member to provide a limited amount of articulation, i.e., compression before making contact with both surfaces to stop the articulation resulting from an excessive load. The helical slot and the area surrounding thering54 are adapted for soft tissue ingrowth while the open core is adapted to accommodate bone and/or soft tissue ingrowth.
• FIGS. 35-37 illustrate a modifieddisc56 in which asuitable polymer56ais enclosed by end plates ordiscs56band56cwith an opencentral core56dfor accommodating a bones strut to provide soft fusion. The spacer must have sufficient strength and stiffness (as discussed earlier) to support the adjacent vertebrae in their natural separated setting and yet under normal loads compress sufficiently to disrupt the bone struts within the open core to form one or more fibrous nonunion joints.
• An additional two part disc, suitable for creating soft fusion, is illustrated inFIGS. 38-51 whereFIGS. 38-42 show the bottom orbase component60,FIGS. 43-47 show the top orupper component62, andFIGS. 48-51 show the assembled disc. The base component comprises upper andlower rings60aand60cseparated via apartial ring60dwhich is joined by bridgedportions60eto the upper and lower rings, as shown (FIGS.42,43). Aserpentine slot60dextends through the base component accommodating the infusion of tissue and allowing limited axial and rotational motion between the upper and lower rings. Bottom andtop walls60fand60ginclude centrally locatedconcave portions60′fand60′gfor engaging the exposed surfaces of the supported vertebral bodies. A pair oftubular posts60h,defining open cores60i,extend upwardly from thelower ring60c.The outer surfaces of the rings define a peripheral wall60j(FIG. 41) conforming generally to the kidney shape of the face of the vertebral bodies to be supported. Acentral section60ksurrounding the posts extends upwardly from the lower ring to an open top. Abeveled surface60mis formed at the upper end of the central section to provide a seat for aplate62aof the top component to be described. In addition, an annular bevels60lis formed on the outer surface at the top of tubular posts as shown to mate with a matching bevel on nipples carried by the upper component to limit axial motion as is illustrated inFIG. 51.
• Theupper component62, shown inFIGS. 43-47, includes acover plate62acontoured to mate with the open end ofsection60bvia a matchingbeveled surface62b.The plate is formed withcircular openings62cfrom which dependtubular nipples62ddefining stepped openings, theupper portion62ethereof transitioning to alower portion62fvia a bevel62g.The inner surface of thelower portion62fis arranged to encircle the outer wall of arespective post60hwith the bevel62gbeing arranged to engage the post bevel62lto limit the axial travel of the disc as is illustrated inFIG. 51. Theposts60hand thesupplier62dare sometimes referred to as an appendage The base and upper components as assembled are secured together along thebeveled surfaces60mand62b,for example, by a TIG welding operation to fill in the area between the beveled surfaces. SeeFIG. 48. In the unlikely event of flexural element failure, the device will collapse and allow the beveled edges62gand62lto contact and center the device, limiting motion while stabilizing the disc.
• The alignedopenings60iand62eform bone accommodating channels to enable pseudoarthosis struts to form therein, which along with the mechanical characteristics of the disc, provide soft fusion as discussed.
• FIGS. 52-64 illustrate another two part disc in whichFIGS. 52-56 show an upper (or inside) component;FIGS. 57-61 show a lower (or outside) component andFIGS. 62-63 show the assembled unit. Theupper component64 also comprises an upper andlower ring61aand64cwith an intermediatepartial ring64b,separated from the upper and lower rings, by a serpentinetissue accommodating slot64dand joined thereto by bridgedsegments64e.As was discussed with respect toFIGS. 38-50 the slot accommodates the infusion of soft tissue and allows limited axial and rotational motion. Atop wall64fincludes keels64gand ahollow post64h(defining a bone accommodation channel64i) extends downwardly from the central section of the top ring and defines a notchedkeyway64jin the bottom peripheral wall for cooperating with a mating upwardly projecting key formed on encompassing sleeve of the lower or outside component to be described for limiting the rotational mobility of the disc.
• Thelower component66, shown inFIGS. 57-61, is formed with a base66asupporting a pair of outwardly projecting spaced keels66b,offset 90 degrees from the upper component keels, as is shown inFIG. 64 and an upwardly extendingsleeve66carranged to surround thepost64hin the assembled condition. The base includes a radially inwardly projecting key66dfor mating with thekeyway64j.SeeFIG. 64. In the assembled condition, the two components are secured together, e.g., by welding theouter edge66eof the base66ato theinner edge64kof thering64cas is indicated at65 onFIG. 64.
• FIGS. 65-68 illustrate another embodiment of ananterior disc68 in the general shape of a natural disc (e.g., kidney-shaped) as shown with pinched sides forming a relativelynarrow midsection68a,resulting in expanded or widened front andback wall areas68band68c,respectively (in a horizontal cross-sectional view), and a narrower midsection (of theside walls68′a) as is shown inFIG. 65. The disc further includes top andbottom surfaces68dand68eand keels68fextending outwardly from the top and bottom surfaces. A centrally located core68gaccommodates bone growth to form living, not completely formed, bone struts. Interleaved lateral horizontal slots orslits68hand68iextend from the front and back walls, respectively, through the open core, as shown, to accommodate axial and binding loads and the infiltration of soft tissue. Theslots68hand68iterminate a short distance from the back and front walls, respectively. As an example the widths W1, W2 at the front and rear expandedwall areas68band68cand W3 at the narrow waist wall area, may be about 1.4, 1.2, and 0.95 inches, respectively, as is illustrated inFIG. 65. With the above dimensions in mind, the68hand68islots may have a depth of about 0.012 to 0.014 inches and terminate about 0.250 and 0.135 inches from the back and front walls, respectively. The height h1 of the disc will vary depending upon its intended location. For example, h1 may vary between about 0.2 to 0.38 inches. Also, the number of slots may and probably will vary depending upon the height of the disc with the shorter discs having three slots while the higher discs will have five slots, for example.
• As the disc flexes the widened front and back sectionsadjacent wall68band68coverlying the slots, transition from level to level (vertically) compressing the slots these wall areas tend to widen out. This action allows these wider areas to transition the load to the next bend or level without fatiguing the disc material. By the same token, thenarrower mid-section68aallows more bending, but still without causing fatigue failure. The collapse of the slots with or without soft tissue infused therein serves to limit the compression of the disc due to excessive loads inhibiting fatigue failure.
• FIGS. 69-72 illustrate anintervertebral disc70 designed for posterior implantation. The disc (like the previously described discs) is formed of a suitable biocompatible material, such as Ti, stainless steel, etc. The disc includes a bulbous nose section70a,with a threaded blind bore70afor receiving an implantation tool (not shown) and atail section70bwithside sections70cextending between upper and lower vertebral body engaging surface:70dand70e.A central bonegrowth accommodating opening70fis located between the side sections. The disc is elliptically shaped in an elevational and cross-sectional view as is shown inFIGS. 53 and 54. The disc is formed with fore and aft horizontal tissue accommodating slits orchannels70gwhich extend through the nose and tail sections and partially through the side sections as shown. A centrally located slit70hextends through the side sections and into the nose section. The slits allow limited axial and bending motions. A centrally located aperature70jaccommodates the insertion of a wire for forming theslot70hduring the manufacturing operation Thevertebrae engaging surfaces70dand70eare roughened, i.e., forming projecting pyramids, to provide bone attachment friendly surfaces.
• FIG. 73 illustrates the placement of two of theposterior discs70 on the face of an underlyingvertebral body8. As pointed out previously, the voids between and outside of the discs may be filled in with a material inhibiting bone growth.
• FIGS. 74-78 illustrate an additionalintervertebral disc72 designed for posterior insertion which, like the anterior disc ofFIGS. 65-68, is formed with pinched side walls72aat the center thereof and expanded or widened intermediateside wall sections72b(adjacent the front andrear end walls72cand72d) for accommodating higher loads in the wider intermediate sections and increased bending along the center section. As an example, with an overall length of about 1.0″ and a height h of about 0.40″, the widths W3 of the widened areas may be about 0.48″ and the center narrower area W4 about 0.36″ and the width WS of theend72dis about 0.25″. The disc defines a centralized elliptically shaped bone channel72ewithslots72fand72gextending from the front and back, respectively, through the side walls and core72e,but terminating short of the rear and front walls, as shown. The outlet of the slots are tapered at72hto accommodate bending stresses. The top and bottom surfaces72iand72jare formed withgrooves72kto enhance bone attachment. A threaded blind hole72lis adapted to receive the threaded end of an insertion tool.
• While providing various examples of intervertebral prosthetic discs and a method for accommodating the creation of soft fusion within the discs advances the state of this art, we now propose improvements to provide a superior prosthesis and method by filling the channels defined in the intervertebral prosthetic discs with a material which is less stiff than typical cortical bone including some cancellous bone used in the prior art devices. For example, a filler material having a flexural stiffness less than 10-12 GPa will improve the load compliance and flexibility of the intervertebral prosthesis.
• Where a cortical/cancellous bone blend is to be used as the filler material, cancellous bone, which has a GPa of the order of 4 GPa, should comprise at least the predominate, if not, the sole constituent of the blend. This flexural stiffness is reported to be the average for cancellous bone. The use of such softer filler materials in the discs will allow for a more compliant and energy absorbing device even in the absence of a nonunion joint or pseudoarthosis. A softer filler material will in effect alleviate the need for a nonunion or pseudoarthosis by the formation of a more compliant yet stable fusion.
• An interbody disc, in accordance with the present invention, has (a) upper and lower surfaces for engaging the faces of the adjacent vertebral bodies between which a failed natural disc has been partially or wholly removed, (b) an exterior wall and one or more generally vertically oriented continuous or discontinuous channels (c) a sufficient stiffness to support the separated vertebrae in substantially their naturally spaced relationship while allowing limited motion and flexibility when subjected to a predetermined load to alter the distance between the vertebrae and thereby transfer load and energy to the any material filling the channels or voids and (d) a bio-compatible filler material disposed within the channels, the filler material being compliant and softer than cortical bone, e.g., having a flexural stiffness of less than about 10-12 GPa. The filler material combined with the device characteristics—will dampen the loads and energy transfer prior to the device contacting on the internal stops which will then in turn prevent fatigue failure.
• Where human bone is selected as the filler material, cancellous bone is the first choice. As a second choice cancellous and cortical bone can be blended with cancellous bone being the predominate portion of the blend such as a ratio of cancellous to cortical bone within the range of about 80% to 20% and preferably about 60%+ to 40%.
• Other naturally harvested materials (either from the patient, a donor or an animal) suitable for use in the blend can include any substances softer than bone, such as portions of the removed disc. Morselized bone or bone weakened by gamma sterilization is more compliant then cortical bone and may also be useful as a filler material.
• Bone graft substitutes, such as demineralized bone matrix (DBM), calcium sulfate dehydrate (CSD) ceramic-based bone graft extenders, are believed to be satisfactory filler materials. These will have a very low flexural modulus to allow device bending characteristics but will resist compressive forces when contained in a generally vertical channel. Recombinant Human Bone Morphogenetic Protein (rhBMP-2) liquid, Epidermal Growth Factor (EGF) liquid, Platelet Derived Growth Factor (PDGF), Fibroblast Growth Factors (FGFs), Parathyroid Hormone Related Peptide (PTHrp), Insulin-like Growth Factors (IGFs), and Transforming Growth Factor-Betas (i.e., TGF-B1), may also accompany suitable filler materials in order to induce a specific biological response such as bone or soft tissue activity. Another filler material candidate is polyetheretherketone (PEEK) with or without porosity. This synthetic material has mechanical properties very similar to those in cortical bone without porosity and very similar to cancellous bone when used with porosity. In addition, it is highly controllable and stable allowing the disc to be preassembled with the filler material at a factory site.
• FIGS. 79 and 80 disclose, respectively, a prospective view of anelongated disc80 formed from suitable biocompatible material with three circular vertical openings orchannels80a,80band80c,having chamferedentrances80don the top and bottom of the disc. The body of thedisc80 has symmetrical top and bottom plates,80eand80frespectively, withsinusoidal sides80g.
• Between thetop plate80eand thebottom plate80fare a pair ofcantilevered spring plates80hand80iwhich are created by the respective overlappingslots80i,to provide a controlled spring action.
• A transverserectangular opening80kextends across thevertical opening80bto enable bone growth not only from the top and bottom into the disc body through thevertical openings80a,80band80c,but also from either side through thetransverse opening80kthereby providing further securement of thedisc80.
• Finally, a pair of keels80lare respectively positioned on both the top and bottom surface of thedisc80 to provide a frictional contact with the corresponding surfaces of vertebral endplates.
• FIG. 80 discloses a schematic cross sectional view of thedisc80 operatively positioned betweenvertebra81 and82 with a growth of bone into the respectivevertical channels80a,80band80c.Thefiller material83 has induced the bone fusion or growth in respectively each of the vertical channels and, while not shown, also in the transverse channel oropening80k.Thediscontinuities84 or “cracks” are indicative of an active patient or recipient where movement between therespective vertebra81 and82 has maintained openings that facilitate the spring movement ofspring plates80hand80i.
• FIG. 81 represents a cross-sectional view of adisc85 of a general type shown inFIGS. 65-68, however, with only two intermediate spring plates for insertion into this particular location along the spinal cord. Note, the specific vertebral height of the disc and number of spring plates in a disc will depend upon a desired size for a particular location along a spinal column.FIG. 81 is a representation of an actual cross-sectional view of the implanteddisc85 and a portion of a spinal column removed from a spine of a sheep as a test subject.
• The filler material86 was a mixture of cancellous and cortical bone and the bone growth87 is shown with adiscontinuity88 indicative of movement of the spinal column by the sheep.
• Referring now toFIGS. 20 and 21 as just an example of the present invention, the artificialintervertebral disc10aincludes upper andlower surfaces11 and12, respectively, for engaging the faces of adjacent vertebral bodies (such as8 and9 ofFIG. 1) and anexterior wall13aenclosing a vertically oriented core orchannel17. The structure and function of the several axial dampeningplates20 and18 providinglateral channels19 therebetween. The present invention is focused on the channel orcore17 which is filled with a filler material.
• Thefiller material83 may be inserted into thechannel17 at the surgical site by the surgeon or other attending personnel as shown inFIG. 21A. Some of the filler materials, e.g., PEEK, may be stable enough to be inserted at the factory. The number of channels for receiving the filler material is not limited to one and the channels need not be continuous or completely vertical. Potentially preassembled filler materials, such as PEEK, may be placed in, not only the generally vertical channels, but also the generally horizontal channels or voids, in order to dampen the device immediately prior to device contact or at the mechanical stops preventing fatigue failure. PEEK, with or without porosity, is available from Invibio, a wholly owned subsidiary of Victrex plc.
• The specific resulting structure of our filler material within an artificial intervertebral disc after a period of time (six months or more) implanted within a patient will vary depending on the activity of the patient. For example, an older and/or less active patient may have a relatively sold infusion of bone growth, particularly in a central core opening of the intervertebral disc. A younger and/or more active patient will experience more motion applied to the intervertebral disc with appropriate flexion that will create and/or maintain openings or discontinuities in the bone growth and/or soft tissue infusion.
• FIGS. 52-64 illustrate a two part disc in whichFIGS. 52-56 show an upper (or inside) component andFIGS. 57-61 show a lower (or outside) component.FIGS. 62-63 show the assembled unit in a side elevational view andFIG. 64 is a cross-sectional view of the assembled unit. Theupper component64 comprises an upper andlower ring61aand64cwith an intermediatepartial ring64bseparated from the upper and lower rings, by a serpentinetissue accommodating slot64dand joined thereto by bridgedsegments64e.As was discussed with respect toFIGS. 38-50, the slot may accommodate the infusion of soft tissue and allow limited axial and rotational motion. Atop wall64fincludes keels64gand ahollow post64h(defining a filler material accommodating top channel64i) extends downwardly from the central section of the top ring and defines a notchedkeyway64kin the bottom peripheral wall for cooperating with a mating upwardly projecting key formed on encompassing sleeve of the lower or outside component to be described for limiting the rotational mobility of the disc.
• Thelower component66, shown inFIGS. 57-61, is formed with a base66asupporting a pair of outwardly projecting spaced keels66b,offset 90 degrees from the upper component keels, as is shown inFIG. 64A and an upwardly extendingsleeve66c(arranged to surround thepost64hin the assembled condition). The base defines acircular opening66fwhich, together with theopening64j,forms a channel for receiving thefiller material83. The base also includes a radially inwardly projecting key66dfor mating with thekeyway64k.SeeFIG. 64. In the assembled condition, the two components are secured together, e.g., by welding theouter edge66eof the base66ato theinner edge64kof thering64cas is indicated at65 onFIG. 64.
• The prosthesis is completed by filling thechannel64jwith anappropriate filler material83 which is shown inFIG. 64A.
• FIGS. 69-72 illustrate anintervertebral disc70 designed for posterior implantation. The disc (like the previously described discs) is formed of a suitable biocompatible material, such as Ti, stainless steel, etc. The disc includes a bulbous nose section70a,with a threaded blind bore70a,for receiving an implantation tool (not shown) and atail section70bwithside sections70cextending between upper and lower vertebralbody engaging surfaces70dand70e.A central filler material accommodating opening orchannel70fis located between the side sections. The disc is elliptically shaped in an elevational and cross-sectional view as is shown inFIGS. 53 and 54. The disc is formed with fore and aft horizontal tissue accommodating slits orchannels70gwhich extend through the nose and tail sections and partially through the side sections as shown. A centrally located slit70hextends through the side sections and into the nose section. The slits allow limited axial and bending motions. A centrally located aperature70jaccommodates the insertion of a wire for forming theslot70hduring the manufacturing operation. Thevertebrae engaging surfaces70dand70eare roughened, i.e., forming projecting pyramids, to provide bone attachment friendly surfaces. Thechannels70fhave been filled with a selectedfilter material83 as is shown in the schematic perspective view ofFIG. 73A. As shown inFIG. 73A a pair of discs can be positioned on the corresponding surfaces of vertebral endplates and the roughenedsurface70 and70ecan assist in maintaining the respective disc placement while thefiller material83 facilitates the subsequent bone growth.
• It is to be noted that the use of the term “adjacent” vertebral bodies includes the fifth lumbar vertebrae and the sacrum. It is also to be noted that the cross-sectional area of the channels to the total cross-sectional area of the disc may exceed the 35% preferred amount.
• The method of the present invention entails the steps of a) providing a disc of the-type described herein b) filling the channel or channels with one of the filler materials described previously either at the surgical site or elsewhere, and c) inserting the completed disc between selected vertebral bodies.
• There has been described a prosthetic intervertebral disc for restoring the motion between the supported vertebral bodies while enabling the formation of pseudo arthrosistic continuous or discontinuous bone struts having nonunion locations within the disc and between the supported bodies thereby providing a state of soft fusion and optionally accommodating the infusion of soft tissue within generally planar spaces within the disc. The disc may take many structural forms as is illustrated by the accompanying drawings. Variations and improvements to the soft fusion/hybrid disc of the present invention will undoubtedly occur to those skilled in the art without involving a departure from the invention as defined in the appended claims.

Claims (21)

1. An intervertebral prosthesis comprising:

a) an interbody disc having upper and lower surfaces for engaging the faces of adjacent vertebral bodies between which a natural disc is to be partially or wholly removed, an exterior wall and one or more generally vertically oriented continuous or discontinous channels adapted to receive a filler material;

b) the disc having a stiffness sufficient to support the bodies in substantially their natural spaced relationship while allowing limited motion and being flexible enough when subjected to a predetermined load to alter the distance between the vertebrae engaging surfaces and transfer energy to any filler material within the channels;

c) the channel(s) being partially or completely filled with bio-compatible filler material having a flexural stiffness of less than about 12 GPa.

2. The prosthesis ofclaim 1 wherein the filler material has a flexural stiffness is less than about 10 GPa.

3. The prosthesis ofclaim 2 wherein the filler material has a flexural stiffness within the range of about 0-6 GPa.

4. The prosthesis ofclaim 1 wherein the filler material is predominately composed of cancellous bone.

5. The prosthesis ofclaim 2 wherein the filler material comprises about 60% to 70% of cancellus bone and about 30% to 40% cortical bone.

6. The prosthesis ofclaim 5 wherein the filler material comprises cancellous and cortical bone in the ratio of about 80% to 20%, respectively.

7. The prosthesis ofclaim 1, wherein the disc is further arranged to limit the movement thereof in the absence of fused bone within the channel(s) to an amount which is sustainable by the disc without resulting in fatigue failure during an anticipated life span.

8. The prosthesis ofclaim 1 wherein the disc further defines at least one generally horizontally oriented tissue channel extending from the filler material channel(s) through the exterior wall to accommodate the infusion of soft tissue therein.

9. The prosthesis ofclaim 1 wherein the filler material is PEEK with and/or without porosity.

10. The prosthesis ofclaim 1 wherein the filler material comprises 95% to 100% cancellous bone.

11. A method of replacing a damaged or failed natural spinal disc comprising:

a) providing an intervertebral disc having upper and lower surfaces for engaging the faces of the vertebral bodies between which the natural disc is to be partially or wholly removed, the disc having an exterior wall and one or more generally vertically oriented continuous or discontinuous channels adapted to receive a filler material and having sufficient stiffness, in the absence of the filler material to support the vertebral bodies in their spaced relationship while allowing limited motion and being flexible enough when subjected to a predetermined load to contract and transfer energy to the filler material within the channels;

b) filling the channels at least partially with a bio-compatible material having a flexural stiffness characteristic less than 10-12 GPa to form a completed prosthesis; and

c) inserting the prosthesis between the vertebral bodies between which a natural disc has been removed in whole or in part.

12. (canceled)

13. An intervertebral prosthesis comprising:

a) an interbody disc having upper and lower surfaces for engaging the faces of adjacent vertebral bodies between which a natural disc is to be partially or wholly removed, an exterior wall and one or more generally vertically oriented continuous or discontinuous channels adapted to receive a filler material;

b) the disc having a stiffness sufficient to support the bodies in substantially their natural spaced relationship while allowing limited motion and being flexible enough when subjected to a predetermined load to alter the distance between the vertebrae engaging surfaces and transfer energy to any filler material within the channels;

c) the channel(s) being partially or completely filled with bio-compatible filler such as cancellous bone or of an equivalent material of stiffness being less than cortical bone yet stiffer than soft tissue or a pseudoarthrosis.

14. An intervertebral prosthesis comprising:

an interbody disc having upper and lower surfaces for engaging the faces of adjacent vertebral bodies between which a natural disc is to be partially or wholly removed, an exterior wall and one or more generally vertically oriented continuous or discontinous channels adapted to receive a filler material, the interbody disc includes a plurality of generally sinuous oriented tissue channels formed by spaced openings extending across the interbody disc to provide a predetermined flexibility to receive compression and binding loads and to accommodate an infusion of soft tissue therein, the resilient compression and expansion of the tissue channels provide a connective transport of nutrients and waste material to and from the interbody disc when in situ,

wherein the channel(s) being partially or completely filled with bio-compatible filler material having a flexural stiffness of less than about 12 GPa, and the interbody disc having a stiffness sufficient to support the adjacent vertebral bodies in substantially their natural spaced relationship while allowing limited motion and being flexible enough when subjected to a predetermined load to alter the distance between the vertebrae engaging surfaces and transfer energy to filler material within the channels.

15. The intervertebral prosthesis ofclaim 14 wherein sinuous oriented tissue channels are formed in a continuous solid disc body member with spaced horizontal openings extending from a first side of the external wall to an opposite second side of the external wall.

16. The intervertebral prosthesis ofclaim 15 wherein the first side and second side of the external wall are offset by 90 degrees from a third side and an opposite fourth side of the external wall.

17. The intervertebral prostheses ofclaim 14 wherein a first set of sinuous oriented tissue channels are formed in a disc body member and a second set of sinuous oriented tissue channels are formed in the disc body member apart from the first set.

18. The intervertebral prostheses ofclaim 17 wherein one or more generally horizontal oriented continuous or discontinuous channels are adopted to receive a filler material to induce bone fusion or growth in each continuous or discontinuous channel.

19. The intervertebral prostheses ofclaim 17 wherein a generally horizontally oriented channel extends between and separates the respective first and second sets of sinuous oriented tissue channels.

20. The intervertebral prostheses ofclaim 19 wherein a plurality of vertically oriented continuous channels extend between the upper and lower surfaces of the interbody disc including a separate continuous channel through respectively, the first and second set of sinuous oriented tissue channels.

21. The intervertebral prostheses ofclaim 14 where three vertically oriented continuous channels extend between the upper and lower surfaces of the interbody disc to induce bone fusion or growth in each continuous channel.

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